1. Field of the Invention
The present invention relates to a super wide angle lens system which can be used for a monitoring camera (CCTV) etc.
2. Description of the Related Art
In general, a super wide angle lens system is used as the lens system in a monitoring camera or the like, to view or monitor a wide angle range. In order to increase the back focal distance and widen the angle of view, a retrofocus type of super wide angle lens having a negative front lens group and a positive rear lens group is used. In such a retrofocus type, the angle of view can be widened by increasing the negative power of the front lens group. To this end, a plurality of negative lens elements of the rear lens group share the negative power. Generally, the negative lens elements consist of a negative meniscus first lens element with a convex surface facing the object side and a negative second lens element. The meniscus lens can advantageously reduce, due to the shape thereof, the astigmatism and distortion of a bundle of light chiefly at a large angle of view. It is mainly for this reason that the meniscus lens has been used as the negative first lens element of the front lens group.
In a super wide angle lens system having an angle of view in the range of 120.degree. to 140.degree. and in which the front lens group consists of a negative meniscus first lens element and a negative second lens element, the radius of curvature of a second concave surface (surface on the image side) of the negative meniscus first lens element is reduced (i.e., the depth of the concave surface is increased). However, this makes it very difficult to produce the meniscus lens. If the negative power of the second lens element is increased, the negative power of the first lens element is reduced. Consequently, the radius of curvature of the second surface of the first lens element is increased. However, if the second lens element is made of a biconcave lens to increase the negative power, an under curvature of field occurs. In order to solve this problem, upon design, consideration must be given to balance the negative power between the first lens element and the second lens element.